Dog poo wrapped in plastic bags discarded by careless owners has emerged as one of the biggest single threats to the health and safety of beach visitors, a marine conservation charity has warned.

The finding is in the latest beach litter data collected by the Marine Conservation Society (MCS) last year and published on Thursday in its Beachwatch Big Weekend Report.

The charity says the volume of dog excrement in bags left on UK beaches rose 11% between 2010 and 2011. Scotland recorded the biggest increase – 71% in one year.

MCS Beachwatch officer Lauren Davis says the findings reveal good and bad habits: "We're delighted that pet owners enjoy dog-friendly beaches and clearly think ahead by carrying poop-scoop bags. But we hope our findings will now encourage them to take the bag off the beach and bin it in one of the many receptacles provided for the job.

"Leaving a bag full of poo on the beach will result in preserved excrement, protected from the elements for years by a bag which could take a long time to break down."

She said it also presented a major health risk to humans: "We don't want children picking up bags that break open and spill their contents whether it's fresh or 'mature'. Dog poo is a source of high levels of bacteria and can lead to reduced water quality, and also poses a human health risk."

Despite an increase in poop-scoop bags, overall shore litter dropped by 11% between 2010 and 2011. The charity says it hopes this will be the start of a downward trend. From an all-time high in 2008, it fell in 2009 and then rose again in 2010.

Davis continued: "The latest results from our weekend-long Beachwatch event held on 17 and 18 September last year are more encouraging than they have been for a while. Not only did beach litter drop overall, we also saw a substantial dip of 33% in the amount of sewage-related debris (SRD) on our beaches – that's the stuff people put down their loos but shouldn't, like cotton buds, condoms, sanitary towels and tampon applicators.

"In 2010 there was a 40% rise in SRD compared to the previous year, but after we'd highlighted the issue and urged people to change their habits, the latest data looks like the message may be getting home."

The MCS Beachwatch Big Weekend 2011 results were collected by almost 4,500 volunteers, who cleaned 335 beaches, covering a total of 88 miles (142.3km), collecting 247,914 items of litter filling 2,177 bags. For every kilometre surveyed almost 1,741 pieces of litter were found.

The MCS said it was also concerned by a rise in the number of balloons found on UK beaches, increasing by 8% since 2010.

"With 2012 set to be a year of celebrations from the Queen's diamond jubilee to the London Olympics, we really need people to understand why letting go is a bad idea. There is clear evidence that balloons harm wildlife in the marine environment and we don't want to see 2012 leaving a legacy of littering," said Lauren Davis.

MCS said litter levels dropped in 2009 from an all-time high in 2008 and rose again in 2010. The charity said it hoped the drop of 11% last year would be the start of a downward trend.

The next major public beach clean-up in the UK is being organised by the MCS in conjunction with Marks & Spencer on 150 beaches at the end of May, while a further Beachwatch Big Weekend will take place in September this year.

Davis added: "Litter levels on our beaches are still are worryingly high. Our September 2011 Beachwatch Big Weekend saw volunteers take to the beaches in driving wind and rain. In September 2012 we would like to see more volunteers and more beaches being cleaned to give us an even clearer picture of the state of our UK beaches."

• This article was amended on 22 March 2012. The original said that the MSC is organising a major public beach clean-up in conjunction with Marks & Spencer. That should have been MCS and has been corrected.